BT has been criticised by Ofcom for overcharging TalkTalk and Sky Broadband for engineering work.
Both TalkTalk and Sky had insisted that BT's Openreach infrastructure division had overcharged them when engineers were deployed to resolve faults on the network.
Ofcom has now concluded that this was indeed the case, with BT billing more than the actual time worked by engineers.
BT has therefore been ordered to repay Sky and TalkTalk the amounts overcharged with interest.
"We have identified the amount by which BT has overcharged for each service in each year," Ofcom said.
"We do not have accurate information as to the volumes of each service purchased by TalkTalk and Sky and therefore have left it to the parties to agree the exact levels of repayment that are due, based on our calculated unit charges."
The dispute could provide further ammunition to Sky and TalkTalk as they lobby for a full legal separation of BT and Openreach, which Ofcom has stopped short of demanding thus far.